Many automotive chassis, body structural members and the like have been hitherto manufactured by subjecting steel sheets having predetermined strength to press working. In recent years, from the viewpoint of global environmental protection, there have been strong demands for reduction in weight of automotive bodies, and there have been continued efforts to increase the strength of steel sheets to be used so that the thickness thereof can be decreased. However, press workability decreases with increasing strength of steel sheets and, therefore, it is often difficult to form steel sheets into a desired member shape.
Accordingly, UK Patent No. 1490535 proposes a working technique referred to as “hot pressing” in which, using a metal mold including a die and a punch, a heated steel sheet is subjected to working and is simultaneously rapidly cooled. That working technique can achieve both facilitation of working and an increase in strength.
However, in hot pressing, since the steel sheet is heated to a high temperature of about 950° C. before hot pressing, scale (iron oxides) is formed on the surface of the steel sheet, and the scale is separated during hot pressing, thereby damaging the metal mold. The hot pressing also has a problem in that scale damages the surface of a member which has been hot-pressed. Furthermore, the scale remaining on the surface of the member may cause poor appearance and degradation in coating adhesion. Consequently, in some cases, the scale on the surface of the member may be usually removed by a performing treatment such as pickling or shot blasting.
However, such removal of scale complicates the process of manufacturing the hot-pressed member and decreases productivity.
Furthermore, automotive chassis, body structural members and the like also require excellent corrosion resistance. In the hot-pressed member manufactured by the method including the step of removing scale as described above, since a rust preventive film such as a coating layer is not provided, corrosion resistance of the member is insufficient.
For this reason, there has been a need for a hot-pressing technique capable of suppressing formation of scale during heating of a steel sheet before hot pressing and improving corrosion resistance of a member after hot pressing. In response to this need, a hot-pressing method has been proposed, which uses a coated steel sheet having a coating layer provided on the surface of a steel sheet. For example, Japanese Patent No. 3663145 discloses a method in which by hot-pressing a steel sheet coated with Zn or a Zn-based alloy, a Zn—Fe-based compound or a Zn—Fe—Al-based compound is formed on the surface thereof, thereby enhancing corrosion resistance of the hot-pressed member.
Furthermore, Japanese Patent No. 4329639 discloses a technique that suppresses liquid metal embrittlement cracking that occurs when molten Zn contained in a coating layer enters a substrate steel sheet during high-temperature heat treatment of the steel sheet before hot pressing. Specifically, JP '639 discloses a steel sheet for heat treatment that includes a Zn coating layer provided on the surface of a steel sheet having a predetermined steel composition, the Zn coating layer being composed of an Fe—Zn alloy having an Fe content of 13% to 80% by mass and an Al content of 0.4% by mass or less and having a Zn coating weight of 10 to 65 g/m2.
In the hot-pressed member manufactured by the method described in JP '145, since a zinc-coated steel sheet or a zinc-aluminum coated steel sheet having a low melting point is used, zinc in the coating layer may enter the steel sheet during hot press forming and causes cracking (liquid metal embrittlement cracking).
Furthermore, even when the coated steel sheet described in JP '639 is used, in some cases, liquid metal embrittlement cracking may occur depending on the hot-pressing conditions of heating temperature, hot-press-forming starting temperature and the like. It has not been possible to completely avoid liquid metal embrittlement cracking.